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The riddle of the forbidden UV absorption of aqueous nitrate: The oscillator strength of the n → π* transition in NO3 − including second order vibronic coupling

Abstract

The environmentally relevant n → π ∗ transition in the nitrate anion is doubly forbidden by symmetry, as the Franck-Condon and first order vibronic coupling terms are both zero in the gas phase. Inclusion of the second order vibronic coupling term is therefore essential when calculating the oscillator strength. Here we have calculated an oscillator strength of 5.7 · 10 − 6 . The second order vibronic coupling term is included by manually displacing the ground-state geometry simultaneously along two normal modes, Q l and Q k , in 19x19 steps, and calculating the transition dipole moment at each point by TD-DFT/ ωB97XD/aug-cc-pVTZ and fitting to a polynomial in order to evaluate the second derivative with respect to Q l and Q k . In the aqueous phase the high symmetry of NO3 − is broken and the first order term is no longer forbidden. Surprisingly the calculated first order term does not contribute significantly to the overall oscillator strength of 1.9 · 10 − 6 . This is a rare example of higher order vibronic coupling being more important than the lowest order term.

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Publication details

The article was received on 04 Jul 2019, accepted on 04 Sep 2019 and first published on 05 Sep 2019


Article type: Paper
DOI: 10.1039/C9CP03774H
Phys. Chem. Chem. Phys., 2019, Accepted Manuscript

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    The riddle of the forbidden UV absorption of aqueous nitrate: The oscillator strength of the n → π* transition in NO3 − including second order vibronic coupling

    P. D. Pedersen , M. H. Rasmussen, K. V. Mikkelsen and M. S. Johnson, Phys. Chem. Chem. Phys., 2019, Accepted Manuscript , DOI: 10.1039/C9CP03774H

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